Polyester felts, usually melt blown, are known to be useful filter materials for applications such as bag filters. These materials are relatively inexpensive, but have relatively large pore sizes because of the diameter polyester threads necessary to result in a sufficiently strong filter material. U.S. Pat. No. 5,205,938 suggests a polyester filter material that is improved by providing a graded pore size. But the porosity of the resultant material can not be as low as what is desired because a high density of fibers is needed to provide sufficiently small volumes between fibers. Very small diameter fibers could theoretically be used to achieve a small pore size with similar densities, but such small fibers are exceedingly difficult to produce and handle. This density, or porosity, results in a greater pressure drop that is desirable.
U.S. Pat. No. 5,318,831 suggests that a small pore size can be achieved by laminating a needle layer to a nonwoven fabric sheet. The needle layer is made of ultra fine fibers. Significant strength is imparted by the nonwoven fabric sheet. A small pore size is still achieved in a felt type of material because of the ultra fine fibers, but the filter material has a greater than desirable density.
Another commercially available filter material consists of a membrane of polytetrafluoroethylene resin laminated to the surface of a felt substrate. This type of membrane can have very small pore openings. The resin membrane is stretched to form small voids within the membrane. The voids can be very small, but the void volume of the membrane is not large, and a significant pressure drop is incurred by a stream passing through the membrane. Further, this type of filter material is relatively expensive, and the resin membrane is prone to breakage.
It would be desirable to provide a filter material wherein a high porosity can be achieved along with a small pore size in a filter layer, thereby minimizing pressure drop through the filter material.
Polymeric open cell foams are used in air and water filters in applications such as air filters for lawn mowers and aircraft. Some of these filters, for example, BRACKET.RTM. aircraft air filters, include a plurality of layers of decreasing pore size foam. These filters trap particles within the volume of the foam, and therefore it is difficult to remove the particles from the filter. These filters are not strong enough to be used as filter material in, for example, bag house filters.
Low density foams are porous crosslinked polymer blocks. Such low density porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion having relatively small amounts of a continuous oil phase, the oil phase containing polymerizable monomers, and relatively greater amounts of an internal water phase.
Such low density foams are prepared by a process disclosed in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking monomers in the continuous oil phase of a high internal phase water-in-oil emulsion with a polymerization initiator such as potassium persulfate. Generally, these high internal phase water-in-oil emulsions contain at least 90 weight percent of a water phase as the internal phase. The high ratio water-in-oil emulsions are formed by combining the oil phase with water under moderate shear. In order to obtain this high internal phase water-in-oil emulsion, a surfactant is used to stabilize the emulsion. It is also advantageous to incorporate an electrolyte into the aqueous phase. The amount and type of electrolyte, along with the amount and type of surfactant, effects the pore size, and hydrophobic/hydrophilic character of the cured foam.
Composite foams are disclosed in U.S. Pat. No. 5,037,859. These foams are prepared by first preparing a rigid foam, the rigid foam being a rigid open cell foam, such as a polystyrene foam. The first rigid foam has a relatively large pore size. A second emulsion is then forced into the first rigid foam to form a smaller pore diameter foam within the polystyrene. The resultant composite foams have greater strengths and densities then foams produced from only the second emulsion. The resultant composite foams also retain the better wicking properties of the first foam, but the resultant composite foam is stiff, inflexible, and brittle. It is therefore not an acceptable material for uses such as bag house filters.
It is therefore an object, in one aspect of the present invention, to provide a method to prepare filter materials using a flexible substrate, and a foam cured from high internal phase emulsions impregnated in the felt and/or as a layer along with the substrate. In another aspect, it is an object to provide a filter material having a flexible substrate impregnated with a foam material, the foam material having a mean average pore size in the range of about one to about 100 .mu.m (100 microns). In another aspect of the present invention, it is an object to provide a filter material by the method of the present invention. In another aspect of the present invention it is an object to provide such a filter material having, in the portion of the material that is foam, an average pore diameter in the range of about one to about 100 .mu.m (100 micron) and a density of less than about 0.2 gm/cc.